Bovine Somatotropin: Physiology, Lactational Responses and Implications for the Dairy Industry

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Abstract
Somatotropin is a polypeptide hormone of the anterior lobe of the pituitary gland. It consists of 191 amino acids and its molecular weight is 22,000. Blood concentrations of somatotropin reflect balances between release, degradation and binding to receptors. Somatotropin release is under the control of two hypothalamic polypeptides: somatotropin releasing factor and somatostatin. Administration of releasing factor and immunization against somatostatin may be alternatives to exogenous somatotropin for increasing production and efficiency. Somatotropin increases mild yield by 10 to 40% without affecting milk composition so long as the feeding program provides adequate energy and protein. Additional nutrients are derived from increased feed consumption and by diversion from body tissues to milk synthesis. Lactational efficiency is improved by diluting maintenance requirements and by directing absorbed nutrients to milk. Somatotropin causes a major redirection of nutrients from tissues to milk synthesis but mechanisms are not understood well. Somatotropin probably does not act upon the mammary gland directly. Some of the response may be mediated by somatomedins or other polypeptide hormones. Management and nutrition will determine whether improvements possible with somatotropin are realized. Diets normally fed during the early stages of the lactation cycle will be fed for longer periods. In order to gain acceptance, somatotropin must not compromise animal health, milk quality or the safety of milk for human consumption. Impacts upon DHIA records and sire-proofing will depend on how rapidly and widely somatotropin is used. Wide-spread adoption will mean that records will simply be raised to new levels. Sire-proofing will need to be monitored closely to detect abuses. Increased milk yields mean that fewer cows will be needed. Technological advances in the last 25 years have decreased the cow population and number of farms. This trend will continue even if somatotropin is not available commercially. Some dairymen may use somatotropin to maintain total production with fewer cows. In general, larger farms realise greater financial returns from favorable opportunities than smaller farms. However, the expected impact of not adopting somatotropin technology is decreased probability of survival and lower financial performance of both large and small farms.